Method of maintaining cleanliness of substrates and box for accommodating substrates

ABSTRACT

A method of maintaining cleanliness of substrates including a first step for accommodating at least a piece of substrate having a gaseous impurity-trapping filter arranged close thereto in a hermetically sealed box, and a second step for circulating the atmosphere in the box at a rate of two or more times a minute so that impurities in the atmosphere are adsorbed by the gaseous impurity-trapping filter. A box for accommodating substrates includes a housing in which space for accommodating the substrates is hermetically closed with a lid, a gaseous impurity-trapping filter arranged in the housing and adapted to adsorb impurities contained in the atmosphere in space, and an atmosphere-circulating device having a ratio of the circulating capacity to the space volume of not smaller than 2 in order to circulate the atmosphere so as to pass it through the gaseous impurity-trapping filter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for removing detrimentalchemical impurities adhered to substrates to maintain necessary highcleanliness, and to a box for accommodating the substrates to store andtransfer. The substrates include ones such as semiconductor wafers thatare between processing machines in manufacturing semiconductor devices,substrates on which LCD pattern is formed, substrates for photo-masking,and magnetic discs.

2. Discussion of the Background

As semiconductor devices become finer- and denser-packed, processingbecomes more complicated. As a result, adverse affects of contaminationdue to gaseous impurities generated in the process become conspicuous.For instance, in the dry etching process or the CVD (Chemical VaporDeposition) process for metallic wiring, acid gases of high vaporpressure such as chlorine (Cl) and fluorine (F) compounds remain onsemiconductor wafers to corrode the metallic wiring on the semiconductorwafers. Further, in the deep-UV lithography processes, basic gases suchas ammonia or the like in the atmosphere or adsorbed by thesemiconductor wafers cause an abnormality of pattern in machining theresist. These adverse influences due to the gaseous impurities onprocess performance cause problems.

In the existing manufacturing method of semiconductor devices, to secureand maintain cleanliness of the atmosphere in a clean room in which thesemiconductor wafers are processed, a gaseous impurity-trapping filtersuch as a chemical filter or activated carbon filter is provided to anair conditioner to trap and remove efficiently these detrimental gaseousimpurities. By employing such a method, the detrimental gaseousimpurities in the process are prevented from causing contamination.

However, when transferring semiconductor wafers from one processingmachine to another processing machine or from one room to another roomin a clean room, and from one machine to another machine laid betweenclean rooms, as a clean space isolated from the space inside or outsidethe clean room, hermetically sealed boxes are in use. Accordingly, whentransferring wafers from one process to another process, in thehermetically sealed box, due to out gases from the wafers,concentrations of these gaseous impurities become high. As a result,these gaseous impurities brought in by the wafers themselves contaminatethe clean room.

Japanese Patent Application No. HEI 6-87964 discloses a method forcleanly isolating semiconductor wafers from contaminated space outside aclean room. In the method, semiconductor wafers are accommodated in abox of which atmosphere is the same with that of the clean room, and thebox is transferred from a clean room to another clean room.

In the box constituted for the inside thereof to communicate with theoutside atmosphere, a particle-trapping HEPA (High Efficiency ParticleAir) filter or ULPA (Ultra Low Penetration Air) filter and a gaseousimpurity-trapping chemical filter are disposed. In the box,semiconductor wafers and substrates for LCD, which are objects to beprocessed, are accommodated and transferred. However, in this method,the chemical filter filters the atmosphere always containing impuritiesto supply into the box. Accordingly, there is a problem that thechemical filter has very short life.

Further, Japanese Patent Laid-open Publication (KOKAI) No. HEI 8-148551discloses a hermetically sealed case for accommodating substrates thatis provided with filters that trap gaseous impurities at a gas inlet andoutlet where the atmospheric gas goes in and out. However, the flow rateof the gas in the atmosphere, being restricted to approximately 1 1/min,is impractical. For instance, when a volume of 23 1 of a box for 8-inchwafer is replaced by the atmospheric gas to trap impurities, under sucha flow rate, more than 20 min is required. Accordingly, impurities thatare brought in the box from the external atmosphere duringopening/closing to insert the wafers are adhered to the wafers. Theimpurity gas, being a base such as ammonia, is adsorbed by resist on thesurface of the wafer to react to the resist, and being an acid such assulfur oxide and chlorine, is absorbed by metallic wiring on the surfaceof the wafer to corrode the metallic wiring.

SUMMARY OF THE INVENTION

The present invention is carried out considering the aforementionedcircumstances. An object of the present invention is to provide a methodby which in a closed space such as a clean room gaseous impuritiesbrought in by substrates such as semiconductor wafers or the like areefficiently removed to maintain an atmosphere of high cleanliness.Another object of the present invention is to provide a box foraccommodating these substrates.

The present invention involves a method that in transferring substratessuch as semiconductor wafers from one processing machine to anotherprocessing machine in a clean room or laid across between clean roomsand in storing these, can maintain cleanliness of the substrates such assemiconductor wafers. That is, with a box comprising a clean space, thesubstrates such as the semiconductor wafers are accommodated in theclean space, transferred and stored. The clean space, isolated from thespace inside or outside the clean room, involves of circulating airgenerated by a filter and a ventilating fan.

A method of maintaining cleanliness of a substrate of the presentinvention comprises a first step of accommodating at least one substratewith a gaseous impurity-trapping filter disposed close thereto in ahermetically sealed box, and a second step of circulating an atmospherein the box at a rate of two times or more per minute so that impurity inthe atmosphere is adsorbed by the gaseous impurity-trapping filter.

According to the present invention, in the second step, the atmospherein the box is preferably circulated at a rate of from two to five timesper minute. In the box, to the substrate, a particles-trapping filter isdisposed in the further close vicinity.

An atmosphere circulating apparatus circulates the atmosphere in thebox. The atmosphere circulating apparatus is intermittently operated asthe need arises.

As the substrate of the present invention, for instance semiconductorsubstrates or glass substrates for mask process can be cited.

The present method for maintaining cleanliness of the substratescomprises a third step of transferring the box to a prescribed positionwhile maintaining cleanliness of the atmosphere in the box.

The box for accommodating substrates of the present invention comprisesa housing in which space for accommodating the substrate is hermeticallyclosed with a lid, and a gaseous impurity-trapping filter disposed inthe housing and adsorbing impurity contained in an atmosphere of thespace, and an atmosphere circulating apparatus having a circulatingcapacity to circulate the atmosphere through the gaseousimpurity-trapping filter, a ratio of the circulating capacity to thespace volume being two or more, preferably from two to five.

In the present invention, the gaseous impurity-trapping filter is formedin pleat, in honeycomb, in sponge, in particle or in plane shape. Thepresent box for accommodating the substrates is provided with a portablepower supply. The portable power supply is attached to the outside ofthe housing.

In the present box for accommodating the substrates, the atmospherecirculating apparatus is intermittently operated.

Inside of the present box for accommodating the substrates comprises aparticles-trapping filter between the gaseous impurity-trapping filterand the substrates.

In the present invention, as the substrate, semiconductor substrate andglass substrate for mask processing can be cited.

According to the present invention, in steps requiring high cleanlinesssuch as semiconductor wafer processing steps, the semiconductor wafer orthe like can be transferred and stored in the efficientlyhermetic-sealed box, in which high cleanliness of the atmosphere ismaintained by use of a gaseous impurity-trapping filter for an arbitrarygaseous impurity and by circulating an inner air by a ventilating fan.In addition, due to intermittent operation of the ventilating fan, anuneven distribution of an impurity concentration in the box can beleveled to result in an efficient replacement of the air in the box in ashort time.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a box for accommodating substrates ofthe present invention.

FIG. 2 is a cross section of the box for accommodating substrates of thepresent invention.

FIG. 3 is a characteristic diagram explaining variation of ammoniaconcentration with time in the atmosphere in the box of the presentinvention in comparison with that of an existing example.

FIG. 4 is a characteristic diagram explaining variation of ammoniaconcentration with time in the atmosphere in the box of the presentinvention comparing between intermittent and continuous operations of aventilating fan.

FIG. 5 is a characteristic diagram explaining variation of ammoniaconcentration with time in the atmosphere in the box of the presentinvention.

FIG. 6A is a perspective view of a pleat like filter used in the box foraccommodating the substrates of the present invention, FIG. 6B being across section of the pleat like filter.

FIG. 7 is a perspective view of a honeycomb filter used in the box foraccommodating the substrates of the present invention.

FIG. 8 is a perspective view of a sponge filter used in the box foraccommodating the substrates of the present invention.

FIG. 9A is a perspective view of a filter of such a type thataccommodates particles in a case, FIG. 9B being a cross section thereof.

FIG. 10 is a diagram showing the relationship between storage time andcorrosion rate of aluminum wiring in an existing box for accommodatingthe substrates.

FIG. 11 is a diagram showing the relationship between number ofventilation and corrosion rates of aluminum wiring in the present boxfor accommodating the substrate.

FIG. 12 is a diagram showing the relationship between amount of organicsubstance adhered to surfaces of a wafer after storing the wafer for aweek and number of ventilation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will be explained with referenceto FIGS. 1 and 2.

FIG. 1 is a perspective view of a box for accommodating substrates ofthe present invention, FIG. 2 being a cross section thereof. Thehermetic-sealed box comprises a box body 1 and a lid 2. The box body 1and the lid 2 are hermetically sealed with a sealing member 3 such as anO-ring or the like of less out gas or, without employing the sealingmember, by use of a processed shape of an overlapping (opening/closing)portion.

The inside of the box body 1 comprises a gaseous impurity-trappingfilter 4, a particle-trapping ULPA or HEPA filter 5, a ventilating fanand a circulation path 7 in which air (a) circulates as shown by arrows.

A wafer-carrier 8 on which wafers 10 are loaded is accommodated in abox, followed by hermetic-sealing. The air in the box body 1 iscirculated by the ventilating fan 6 to go through a chemical filter(impurity-trapping filter) 4, the impurity being trapped, and thecleaned air being sent to the semiconductor wafers 10 further throughthe ULPA filter 5. Thus, the box, while circulating the cleaned airwithin the box and storing the semiconductor wafers 10 in the cleanedair, is transferred from a prescribed step (machine) to another step. Asa power supply 9 for the ventilating fan 6, the box is provided with abattery (secondary battery) (not shown in the figure). Other than duringtransfer, electricity is supplied from the outside. At the same time,the battery is charged.

The air circulation path 7 has a passage consisting of ventilating fan6→ gaseous impurity-trapping filter 4→ particles trapping filter 5→semiconductor wafers 10→ ventilating fan 6, the air flow generated bythe ventilating fan 6 being circulated in this passage.

In the present invention, arrangement of the fan and the filters are notrestricted to that shown in FIG. 1. The ventilating fan and filters canbe disposed anywhere of the box body. Further, number of the fan is notrestricted to one but a plurality of fans can be used as the needarises. In FIGS. 1 and 2, the ventilating fan is disposed below thewafer carrier. For instance at least one of the fans, however, can bedisposed on a side surface of the wafer carrier. When the fan isdisposed on the side surface of the wafer carrier, the filter ispreferable to be disposed below the wafer carrier.

However, even in this case, as shown in FIGS. 1 and 2, the gaseousimpurity-trapping filter is preferable to dispose upstream and theparticles-trapping filter to dispose downstream. That is, it ispreferable to trap particles after trapping the gaseous-impurity.

The box shown in FIGS. 1 and 2 is hermetically sealed after taking inthe air of a clean room. Impurity gases contained in the inhaled air,for instance such as HF, NH₃, HCl, SO_(x) gas or the like are trapped bythe chemical filter 4. Further, particles in the air are trapped by thefollowing HEPA filter or ULPA filter 5. When taking in the air of theclean room, number of particles involved in the air is small.Accordingly, almost all particles can be trapped by the HEPA or ULPAfilter 5 disposed in to the clean box. The air from which the impuritygases and particles are removed is introduced into a space where theobject to be processed such as semiconductor wafers or glass substratesare stored.

As mentioned above, for the filters of the box for accommodatingsubstrates, the gaseous impurity-trapping filter 4 and theparticle-trapping filter 5 are used.

The chemical filter is used for the gaseous impurity-trapping filter 4.For the chemical filter, any one or a combination of an activated carbonfilter for trapping organic gases and chemical filters for trappingharmful chemical impurities each such as a basic gas, an acid gas or thelike can be used.

An ion exchange filter utilizing ion-exchange non-woven fabrics or wovenfabrics or an activated carbon filter utilizing activated carbon fibersobtained by pyrolyzing to activate cellulose fiber, acrylic fiber,lignin fiber is in practical use as gas-trapping filters. An acid oralkali impregnated activated carbon fiber filter or ion exchange fiberfilter can efficiently adsorb and trap ions such as ammonia or the likein the air or ionic substances such as hydrofluoric acid or hydrochloricacid contained in mist.

Next, the activated carbon fiber that is used as base material of thechemical filter will be explained.

The activated carbon is removed of un-textured carbon or the like in theprocess of activation to have lots of fine pores between principalcrystals. Due to the fine pores and large specific surface area, theactivated carbon possesses a large physical adsorption capacity.Utilizing this property, the activated carbon is in practical use.

For the activated carbon filter, there are particulate activated carbonfilters and activated carbon fiber filters, both being able toeffectively trap the gaseous organic substances. In particular, theactivated carbon fiber filter, in comparison with the particulateactivated carbon fiber filter, has finer pores to be resultant largerspecific surface area. Accordingly, as the base material for the airfilter, dust generation is less to be excellent in machinability.

To obtain the activated carbon fiber, first, for instance rayon, kainol,polyacrylonitrile, petroleum, petroleum pitch or the like as rawmaterial is formed into fibers. The fibrous carbon undergoes gasifyingreaction (activation reaction) with water vapor, carbon dioxide or thelike under high temperatures of more than 800° C. With an intention toreinforce strength and to prevent the dust generation, binder that doesnot contribute in adsorption can be added to the activated carbonfibers. The content of the binder or the like is preferable to be scarcefrom a viewpoint of material.

When the particulate activated carbon and activated carbon fibers areimpregnated with chemicals such as an acid or base, a filter capable oftrapping basic or acid impurities in the air can be obtained.

For the chemical filter, the ion exchange filter and activated carbonfilter can be used alone or together.

Next, the particles trapping filter 5 will be explained. For theparticles trapping filter 5, the ULPA filter, HEPA filter or the likecan be used.

The HEPA filter is an air filter having a particle trapping efficiencyof more than 99.97% for particles of a diameter of 0.3 μm under a ratedair flow.

The ULPA filter is developed with an object to obtain theparticles-trapping efficiency higher than that of the HEPA filter,having a particle trapping efficiency of more than 99.9995% forparticles of a diameter of 0.1 μm. Glass fibers is used for filteringmaterial in the ULPA filter. However, polytetrafluoroethylene (PTFE)that does not contain impurities such as boron (B) or metal and is notcorroded by acid, alkali, organic solvent or the like can be used as thefiltering material.

For material of the box, material that evolves less out gas, forinstance polycarbonate (PC) or polybutylene terephthalate (PBT) can beused.

When accommodating air of which ammonia concentration in the air is 5ppb in a box for 8-inch wafers, four times of ventilation can reduce theammonia concentration in the box to less than 1 ppb. In this case, avolume of the box is 23 litters and the flow rate of the air of the fanis 50 1/min, an operation time being approximately two minutes. Further,when the fan is intermittently operated with the ventilation cycle of 30sec operation—10 sec interruption, three times of ventilation cycles areenough to attain 1 ppb. The total operation time of the fan is 1.5 min.When continuously operating the fan, the air in the box flows in adefinite direction. Accordingly, the air stagnated somewhere can not besufficiently drawn out. The interruption of the operation allows thestagnated air to diffuse out of the trapped places to cause animprovement of a ventilation efficiency. Due to the cut of the operationtime, power saving of the battery is accomplished. Further, due to theimpurity concentration in the air and selection of the filter material,the impurity trapping efficiency of the present invention differs alittle. The air of which ammonia concentration is 10 ppb can be purifiedto less than 1 ppb after two minutes operation.

Next, with reference to FIGS. 3 to 5, the variation of the impurityconcentration in the air in the box used in a method of the presentinvention will be explained in detail.

FIGS. 3 to 5 all are characteristic curves showing ammoniaconcentrations in the box, the vertical axis showing ammoniaconcentration (ppb), the horizontal axis showing the operating time ofthe fan (min).

FIG. 3 shows a comparison between the characteristic curves of thepresent invention and the existing technology (Japanese PatentApplication No. HEI 6-87964) that adopts one-pass method involving aclean box of which internal air is not circulated. The fan is operatedwithout interruption. In the one-pass method (existing method), the airthat only once passed the chemical filter is introduced into the box andsent to semiconductor wafers by the fan. By contrast, in the method ofthe present invention, in the hermetically sealed box the internal airis circulated to pass repeatedly the chemical filter. Accordingly, theammonia concentration in the box is gradually decreased. In the figure,a solid line is a curve for the existing one-pass method, a dotted lineshowing the present circulation method. Now consider a case where thetrapping efficiency of the filter is 90% and an environment (ammoniaconcentration) is 10 ppb. Whereas the existing one-pass method decreasesthe ammonia A concentration in the box to 1 ppb, the present methoddecreases gradually the ammonia concentration from 10 ppb to 1 ppb to0.1 ppb and so on by repeating the pass of the filter from one to two tothree times and so on. Thereby, further lower ammonia concentration inthe box can be attained.

FIG. 4 shows a characteristic curve when the fan of the presentinvention is intermittently operated. As obvious from FIG. 4, inintermittently operating, only 10 min operation a day can maintain theammonia concentration in the box at less than a definite value withoutoperating the fan for another 24 hours. Though during 24 hours theammonia concentration in the air in the box increases a little, the next10 min operation of the fan allows maintaining the ammonia concentrationat less than a definite level.

Next, with reference to FIG. 5 the difference of effects of theoperations of the fan with and without interruptions will be explained.The continuous operation decreases the ammonia concentration in the airin the box to 0.1 ppb by continuously circulating the air in the box ofwhich ammonia concentration is 10 ppb. On the other hand, theintermittent operation decreases the ammonia concentration to 0.1 ppb byintermittently circulating the air of which ammonia concentration in theair is 10 ppb with repetition of five times of the cycle of 1 minute ofoperation and 1 minute of interruption. Rather than operating for 10 minwithout interruption, the operation for 10 min with interruption of 1min on and 1 min off can obviously reduce the ammonia concentration tothe same level faster than the continuous operation while further savingpower.

The hermetic-sealing characteristics that the box possesses are definedin the following way. When the ammonia concentration in the externalenvironment is for instance 10 ppb and that in the box is being reducedto 0.1 ppb, if the concentration of at least 0.1 ppb can be maintainedfor a day with the fan off, the box is said to be in a hermeticallysealed state. That is, the hermetically sealing characteristics in thepresent invention is defined as a state where when the box is left for aday an amount of the air getting out of and into the box is less than 1%of the volume.

Next, the life of the chemical filter used in the present invention isexplained compared with that of the aforementioned existing one-passmethod.

Calculation is done based on the operation without interruption. Whencompared the cases of continuously operating without interruption by useof the existing one-pass method and of continuously operating thepresent box assuming that the box is given twenty times ofopening/closing a day, the life of the present chemical filter isapproximately two hundred times longer than that of the existing one.Whereas the one-pass method is operated while always taking in the aircontaining impurity, in the present invention, after opening and closingonce, only the air of the volume in the box is necessary to be filtered.Only the impurity contained in a volume corresponding to (number ofopening/closing)×(the volume of the box) are accumulated in the filter.When the volume of the box is 20 litters (L) and the fan is operated sothat the air in the box is filtered three times per minute, processingamounts of the air per day are as follows;

for one-pass method: 20 L×3 times/min×60 min×24 hours=86400 L

for the present method: 20 L×20 times/day=400 L. Accordingly, when thelife of the filter in the existing one-pass method is one month, that ofthe present invention is approximately 200 months.

Now, when a total amount of ammonia that is generated from semiconductorwafers in the box is 10 ng/min, in a state where the fan is off for 10min, the ammonia concentration in the box becomes 4 ppb. However, due tothe aforementioned operation method the ammonia concentration in the boxcan be always kept under 1 ppb.

Semiconductor wafers treated by buffered hydrofluoric acid (ammoniumfluoride+hydrofluoric acid) and chemically sensitized resist coatedwafers are simultaneously transferred and stored for a prescribed periodof time in the box used in the present method and the existinghermetic-sealed box. Thereafter, when developed, an obvious differenceof width of the resist is found between the two cases. When thesemiconductors are stored in the box to be used in the present method,ammonia that is the out gas from the semiconductor wafers can be trappedand removed to prevent ammonia from adhering to form a layer difficultto dissolve.

The semiconductor wafers after dry-etching are transferred and storedfor a definite period of time in the box to be used in the presentmethod and in the existing hermetic-sealed box to compare. There is anobvious difference in corrosion of wiring between the two cases. Whenaccommodated in the box to be used in the present method, Cl₂ gasremaining with the semiconductor wafers is trapped to enable to preventAl wiring from being corroded due to adsorption of the Cl₂ gas.

A slight amount of organic out gases that are generated from constituentmembers such as the box and sealing material to be used in the presentmethod is trapped by the activated carbon filter in the box. Thereby,the organic can be prevented from adhering to the surfaces of thesemiconductor wafers.

The present invention can be employed, other than in semiconductor waferprocess, in the mask process and liquid crystal process of glasssubstrates.

As the chemical filter, other than ion exchange fibers/non-woven fabric,the chemical filter in which an acid or base is impregnated in theactivated carbon can be used. As the activated carbon filter, a filtercomposed of, other than activated carbon fibers, fine particulateactivated carbon can be used. When a filter having an absorbent capableof efficiently absorbing moisture is used, humidity can be kept low.

Further, with reference to FIGS. 6 to 9, the shape of the chemicalfilter being used for the box for accommodating substrates of thepresent invention will be explained.

FIG. 6A is a perspective view of a pleat filter, FIG. 6B being a crosssection thereof. FIG. 7 is a perspective view of a honeycomb filter,FIG. 8 being a perspective view of a sponge filter. FIG. 9A is aperspective view of a filter that accommodates particles in a case, FIG.9B being a cross section thereof.

According to the present invention, by forming the filter in a pleattype, a honeycomb type, a sponge type or a type that accommodatesparticles in a case, a surface area of the filter can be made larger toenable to reduce the pressure loss due to the filter. As a result, aflow rate of the air in the atmosphere can be increased to for instancemore than 70 litters/min. When replacing for instance 23 litters of thevolume of the box for 8-inch wafers, the air can be circulated threetimes per minute, that is, number of ventilation being three times perminute. The impurities in the atmosphere can be reduced to one tenthwithin 1 minute. The number of ventilation is the number with which theatmosphere in the box is circulated in per minute. When the volume ofthe box is A (litter) and the flow rate is B (litters/min), the numberof ventilation X is expressed by B/A.

According to the present invention, thus the time for trapping theimpurities in the box can be greatly shortened. When the impurity gas isa base such as ammonia, the resist on the surface of the wafer can beprotected from adsorption. When the impurity gas is acid such as sulfuroxide and chlorine, metallic wiring on the surface of the wafer can beprotected from being corroded.

With reference to FIGS. 10 to 12, the number of ventilation provided inthe present invention will be explained.

FIG. 10 is a diagram showing the relationship between the storage timein the existing box and rate of corrosion of aluminum wiring. Accordingto the diagram, when the storage time exceeds approximately 10 min, thecorrosion of the aluminum wiring proceeds rapidly.

FIG. 11 is a diagram showing the relationship between the number ofventilation in the substrate accommodating box and the rate of corrosionof the aluminum wiring. According to FIG. 11, when the number ofventilation is more than two times per minute, the corrosion rate (%) ofthe aluminum wiring becomes 0%. According to the box disclosed inJapanese Patent Laid-open Publication (KOKAI) No. HEI 8-148551, the flowrate being 1 1/min, when the volume is 23 litters, the number ofventilation is 0.04 times per minute, the corrosion rate beingapproximately 40%. When the volume is 8 litters, the number ofventilation is 0.1 times per minute, the corrosion rate beingapproximately 10%. The number of ventilation of more than two times perminute such as in the present invention can completely prevent thecorrosion of aluminum from occurring.

FIG. 12 is a diagram showing the relationship between amount of organiccompounds adsorbed on the wafer surface when the wafer is stored for aweek and number of ventilation. The relationship is obtained under thefollowing assumptions. When 24 wafers are stored in the box of thevolume of 23 litters, organic compounds that are impurities in the aircan be trapped down to 0.01 ppb (due to the out gas from the boxmaterial or the like, the organic compounds can not be completelytrapped). Probability of adhesion of the organic compounds to the waferis assumed to be 1/150. The amount of adhesion of the organic compoundsvaries due to the air flow rate (number of ventilation). A permissibleamount of adhesion of the organic compounds is 10 ng/wf. From FIG. 12,taking the fluctuation into consideration, the number of ventilation ispreferable to be less than 10 times per minute, more preferable to beless than 5 times per minute.

As explained in the above, when the box for accommodating substrates ofthe present invention is used, between steps where high cleanliness isrequired, while locally and efficiently maintaining the inside of thebox in a highly clean atmosphere by use of an arbitrary gaseousimpurity-trapping filter for the arbitrary gaseous-impurity and by useof circulation of the air in the box, the substrates to be treated suchas semiconductor wafers can be transferred and stored. Further, even ifthe waiting time to the next step in the actual manufacturing line isprolonged due to troubles or maintenance of the machine, the substratescan be stored in the clean atmosphere for a long time. Accordingly,without being subjected to an influence of the storage time, a stableprocess characteristics can be obtained and the productivity can beimproved, resulting in being effective as a transferring system of themanufacturing line. When the fan is intermittently operated, unevennessof the impurity concentration in the box is leveled to enable for theair to be replaced efficiently in a short time. Thereby, power isconsumed more efficiently, the power supply being made the minimumrequirement.

While the present invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and detail maybe made therein without departing from the spirit, scope and teaching ofthe invention. Accordingly, the invention herein disclosed is to beconsidered merely as illustrative and limited in scope only as specifiedin the appended claims.

What is claimed is:
 1. A method of maintaining cleanliness of asubstrate comprising: a first step of accommodating at least onesubstrate with a gaseous impurity-trapping filter disposed close theretoin a hermetically sealed box; and a second step of circulating anatmosphere in the box at a rate of two times or more per minute by useof an atmosphere circulating apparatus and interrupting the atmospherecirculating apparatus at a rate of once or more per several seconds toseveral minutes to diffuse the atmosphere, and causing the gaseousimpurity-trapping filter to adsorb impurity in the atmosphere.
 2. Themethod of maintaining cleanliness of a substrate as set forth in claim1, wherein the atmosphere circulating apparatus is interrupted at a rateof once or more per minute to diffuse the atmosphere.
 3. The method ofmaintaining cleanliness of a substrate as set forth in claim 1, whereinin the second step the atmosphere in the box is circulated at a rate oftwo to five times per minute.
 4. The method of maintaining cleanlinessof a substrate as set forth in claim 1, wherein in the box a particletrapping filter is further disposed in close vicinity of the substrate.5. The method of maintaining cleanliness of a substrate as set forth inclaim 1, wherein the substrate is a semiconductor substrate or a glasssubstrate for mask process.
 6. The method of maintaining cleanliness ofa substrate as set forth in claim 1, further comprising a third step oftransferring the box to a prescribed place while maintaining cleanlinessof the atmosphere in the box.
 7. A box for accommodating a substrate,comprising: a housing in which space for accommodating the substrate ishermetically closed with a lid; a gaseous impurity-trapping filterdisposed in the housing and adsorbing impurity contained in theatmosphere of the space; and an atmosphere circulating apparatus thatcirculates the atmosphere so that the atmosphere goes through thegaseous impurity-trapping filter at a rate of two times or more perminute and interrupts at a rate of once or more per several seconds toseveral minutes to diffuse the atmosphere.
 8. The box for accommodatinga substrate as set forth in claim 7, wherein the atmosphere circulatingapparatus in one that circulates the atmosphere so as to go through thegaseous impurity-trapping filter at a rate of two times or more perminute and interrupts at a rate of once or more per minute to diffusethe atmosphere.
 9. The box for accommodating a substrate as set forth inclaim 7, wherein the gaseous impurity-trapping filter is formed in apleat shape.
 10. The box for accommodating a substrate as set forth inclaim 7, wherein the gaseous impurity-trapping filter is formed in ahoneycomb shape.
 11. The box for accommodating a substrate as set forthin claim 7, wherein the gaseous impurity-trapping filter is formed insponge shape.
 12. The box for accommodating a substrate as set forth inclaim 7, wherein the gaseous impurity-trapping filter is formed inparticles.
 13. The box for accommodating a substrate as set forth inclaim 7, further comprising a portable power supply.
 14. The box foraccommodating a substrate as set forth in claim 13, wherein the portablepower supply is attached on the outside of the housing.
 15. The box foraccommodating a substrate as set forth in claim 7, further comprising aparticles trapping filter between the gaseous impurity-trapping filterand the substrate.
 16. The box for accommodating a substrate as setforth in claim 7, wherein the substrate is a semiconductor substrate ora glass substrate for mask process.